Hydrocarbons such as oil, natural gas, etc., may be obtained from a subterranean geologic formation, e.g., a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. This provides a partial flowpath for the hydrocarbons to reach the surface. In order for oil to be produced, that is travel from the formation to the well bore (and ultimately to the surface), there must be a sufficiently unimpeded flowpath from the formation to the well bore. Unobstructed flow through the formation rock (e.g., sandstone, carbonates) is possible when rock pores of sufficient size and number are present for the oil to move through the formation.
However, as is becoming more generally known, greater effort and varied approaches must be undertaken to produce hydrocarbons since the relatively easier to produce subterranean formations have generally been found. Thus, the oil and gas industry is looking at producing hydrocarbons from subterranean formations where recovering the hydrocarbons is more difficult and requires many steps, including the introduction and placement of various components, additives and agents at relatively precise locations downhole.
One such more complicated process involves hydraulically fracturing the subterranean formation—literally breaking or fracturing a portion of the strata surrounding the wellbore. The development of suitable fracturing fluids to provide the necessary hydraulic force is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and high shear rates which can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids which have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used, which may or may not be crosslinked. The thickened or gelled fluid helps keep the proppants within the fluid during the fracturing operation.
While polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles in the brine, such polymers require separate breaker compositions to be injected to reduce the viscosity. Further, the polymers tend to leave a coating on the proppant even after the gelled fluid is broken, which coating may interfere with the functioning of the proppant. Studies have also shown that “fish-eyes” and/or “microgels” present in some polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage. Conventional polymers are also either cationic or anionic which present the disadvantage of likely damage to the producing formations.
Aqueous fluids gelled with viscoelastic surfactants (VESs) are also known in the art. VES-gelled fluids have been widely used as gravel-packing, frac-packing and fracturing fluids because they exhibit excellent rheological properties and are less damaging to producing formations than crosslinked polymer fluids. VES fluids are non-cake-building fluids, and thus leave no potentially damaging polymer cake residue. However, the same property that makes VES fluids less damaging tends to result in significantly higher fluid leakage into the reservoir matrix, which reduces the efficiency of the fluid especially during VES fracturing treatments. It would thus be very desirable and important to discover and use fluid loss agents for VES fracturing treatments in high permeability formations.
Many techniques and compositions are known to introduce chemicals, particles and other agents on a delayed release downhole, not only for purposes of fracturing, but for other reasons, including, but not limited to reducing fluid loss (as mentioned), breaking the gelled fluid, inhibiting scale, inhibiting corrosion, inhibiting hydrate formation, stimulation treatments (e.g. with acids), for cementing, for remedial purposes, etc. Various methods of keeping the chemical, particle or other agent in a form that is ineffective or preserved until delivery or release at the proper locations downhole include microencapsulation, macroencapsulation, incorporation within an emulsion or multiple emulsion, and the like. It would be desirable if other techniques besides these could be devised to provide an alternative or improved downhole delayed agent delivery system.